A thin-film transistor (hereinafter, may be referred to as a TFT in this specification) is a device that can be formed on an insulator substrate made of glass or the like and plays a pivotal role in electronics technology. As a channel layer material for the TFT, an amorphous silicon or polycrystalline silicon is currently most widely adopted. In recent years, a metal oxide semiconductor (hereinafter, may be referred to as an oxide semiconductor or oxide in this specification) has attracted attention as a substitute for the silicon material. The oxide semiconductor TFT has such features that the oxide semiconductor TFT can permit flow of a larger current than an amorphous silicon TFT can, is less costly than a polycrystalline silicon TFT is, and can be manufactured with a minimal variance in properties from other devices. In addition, not only the property of the oxide semiconductor TFT as the channel layer is excellent but also the oxide semiconductor TFT has a feature that it can be formed at temperature close to room temperature. Therefore, the oxide semiconductor is regarded as one of promising candidates for the channel layer material to be employed when a TFT is formed on a so-called flexible substrate such as a plastic film.
In next-generation TFT-use devices, a TFT that is superb in a property of permitting flow of a large current is requested to be manufactured on a large area at a low cost with a minimal variance in the property. A problem is that although an existing oxide semiconductor TFT has an advantage over a silicon TFT in several points, a current value to be fed by the oxide semiconductor TFT is poorer than that to be fed by a polycrystalline silicon TFT.
An oxide semiconductor TFT that has addressed the above problem is disclosed in Patent Literature 1, A disclosed structure is characterized by the adoption of an oxide semiconductor, which exhibits a low resistance, as an active layer (channel layer) and the inclusion of a resistance layer between the active layer and at least one of a source electrode and drain electrode. Owing to the structure, both a large on current and a small off current are attained.